Prior art selective call receiver systems (paging systems) have endeavored to efficiently communicate information to selective call receivers (pagers) while providing for effective battery saving operation of the selective call receivers. The widely accepted Golay Sequential Code (GSC) provides an asynchronous method for communicating message information to selective call receivers and has been described in U.S. Pat. Nos. 4,424,514 and 4,427,980 both issued to inventors Fennell, et al. The GSC protocol provides a preamble signal which initially synchronizes the selective call receivers to a paging signal. One of the several well known preamble signals may be used, each identifying a different group of selective call receivers which are used to decode following message information. Additionally, U.S. Pat. No. 4,860,003 issued to DeLuca, et al. describes power conservation during the reception of a signal, such as the GSC signal, in response to a signal indicating the occurrence of address information.
An asynchronous/synchronous signalling system has been defined by the Post Office Standardization Code Advisory Group (POCSAG). The operation according to the POCSAG signalling system requires selective call receivers to synchronously decode the received signal subsequent to being synchronized to the POCSAG signal. The POCSAG signal and a method for decoding the signal is described in U.S. Pat. No. 4,663,623 issued to Lax, et al. Like GSC, a POCSAG transmission may begin asynchronously with respect to a prior POCSAG transmission, and once the transmission begins, the signal is synchronously decoded until the transmission ends. The POCSAG transmission has at least one batch, each batch beginning with a synchronization signal followed by eight frames, each frame occurring at a predetermined time after the synchronization signal. A selective call receiver synchronized to the signal needs to search for its address in only a predetermined one of the eight frames. In the remaining seven frames, the selective call receiver conserves power by one of the well known techniques of battery saving. However, subsequent transmissions, for example, the POCSAG signals, need not be either bit or frame synchronized to the previous POCSAG transmission. By permitting a subsequent transmission signal to be synchronized only with each transmission, and not by bit or frame synchronization, the subsequent transmission is asynchronously synchronized to the POCSAG signal.
Most paging protocol signals are designed to co-exist with other paging protocol signals. For example, a GSC transmission may be followed by a POCSAG transmission which may be followed by a 5-tone sequential transmission, etc. It is desirable to provide battery saving features for the selective call receiver during the absence of a signal being directed to the selective call receiver. This has typically been accomplished with a preamble signal preceding the message transmission. Both the GSC and POCSAG signals begin with a long preamble signal providing for selective call receiver synchronization before the message transmission and providing for battery saving in the absence thereof. The preamble signal although providing for battery saving and synchronization, decreases the overall system message throughput. During the time taken for a preamble transmission, no message information is being communicated to the selective call receivers, thereby decreasing throughput of the system.
Synchronous selective call receiver protocols eliminate any preamble signal, thereby enabling a more efficient battery savings. A synchronous paging signal is shown in U.S. Pat. No. 4,642,632 issued to Ohyagi, et al. This synchronous signal has selective call receivers divided into one of a plurality of group fields, each group field occurring at a predetermined period and having a predetermined maximum message information capacity. Since the synchronous signal is always being transmitted, no preamble signals are required, and a selective call receiver needs only decode paging information while its preassigned group is being transmitted. However, varying traffic demands (the amount of message information for a group of selective call receivers) may cause the amount of message information for one group of selective call receivers to exceed the maximum capacity of the group field while another group field has available capacity. This causes transmission of idle signals during one group field, while in another group field, numerous messages are being queued because its capacity is exceeded. The throughput of the overall system is decreased because selective overload patterns are generated from the varying traffic levels within different group fields.
Thus, what is needed is a selective call receiver system which selectively sacrifices battery saving improvements afforded by a synchronous paging signal to prevent or to reduce overload occurring within the selective call receiver system.